The Science of Forming

Stamping is No GoodWho Gets the Blame?

Happens all the time—rejected stampings start the finger pointing, and the press-shop owner’s cry for an explanation is answered by a material supplier refusing to accept responsibility. “Thousands of other stampings are made daily by other press shops using the identical material,” says the material supplier. “Therefore, the press/die combination must be faulty.” The press-shop owner counters that the material must be at fault “because similar material from other suppliers results in perfect stampings in my press/die combination.”

The blame game has begun, and when it ends (hypothetically), both the workpiece material and the press/die are off the blame list. Instead, the rejection occurs because the material properties do not match the requirements of the press and die. The supplier and the press shop share the blame because neither has acquired and exchanged the data needed to ensure a successful match resulting in an acceptable stamping.

Metalformers and their material suppliers have struggled to match material properties to the needs of the press/die for several reasons:

1) Metalforming as a system is complex and difficult to understand. Decades ago, the properties of low-strength mild steel and high-strength low-alloy (HSLA) steel were simple, easily measured and well-controlled. Yield and tensile strength, total elongation and hardness readings were sufficient to predict material formability. A material’s hardness predicts its resistance to surface wear, and has little correlation to formability. Instead, the more important forming parameters of sheetmetal forming to track include work-hardening exponent (n-value), strain-rate hardening (m-value), directionality of properties (r-value), forming-limit curves (FLC) and change of microstructure during forming.

2) Unfortunately, much of this characterization and understanding has been confined to the research laboratory and has been slow to be integrated into day-to-day press-shop operations. A tremendous disparity exists in the level of metalforming knowledge between the OEM/Tier 1 stampers at one end of the supply chain and lower-tier stampers at the other end. Similar disparities exist between major mills and smaller service centers and warehouse suppliers.

3) In all analysis projects, disassembly is easy while shops struggle with synthesis. A laboratory can dissect a complex stamping into a number of unique forming components, each capable of being studied in isolation. Engineers can quantify relationships between each forming operation and the controlling material properties, as well as other process variables. The reverse, though, proves difficult. Different forming operations are controlled by different material properties; assigning levels of difficulty to each forming mode requires the amount of deformation required to reach the final shape. The interaction between forming modes initially is unknown and available only after the stamping is completed.

4) We are great at post-mortems but poor at predictions. We can use SPC to chart historical trends and identify production blips in great detail. We rarely, however, can predict the exact behavior of a future press run, even if we know the performance of the previous 10 or 20 runs. Compare this to doctors proficient at identifying the cause of death but unable to accurately predict how long a suggested treatment will extend a patient’s life.

5) One main press-shop rule appears to be, “If it ain’t broke, don’t tinker with it.” At die tryout, we modify the forming process until we achieve an acceptable breakage rate (ideally zero) at die buyoff. The concept of continuing process modifications until attaining a safety margin or a robust process often is viewed as a waste of time and money.

6) Demands are made for the consistency of one or two system inputs, while ignoring the roles played by the other 50 or more system inputs. Some believe that constantly forcing improvements in the consistency of sheetmetal will create a more consistent output of the forming system. This ignores the fact that other inputs may be more inconsistent or affect the system much more strongly than material properties.

7) The massive job of monitoring so many system inputs starts with monitoring system output. Stampers need accurate and sensitive output data to detect the onset of input deviations. They lose such information when the output data are averaged, summarized or otherwise modified for ease of reporting. For example, the average rejects of fenders stamped over an entire shift hides critical information that all rejects occurred in the first 15 min. of the shift. Likewise, an average shift-reject rate of five percent does not help solve a reject rate of 10 percent for the right fender and 0 percent for the left fender.

Stampers often fail to appreciate the complex interactions within the forming system, the system variations, the distribution curves and the lack of system controls. Instead we tend to view the forming system as a black box that directly links input to output on a one-to-one basis. When the black box begins to respond unpredictably, we blame the black box for deviating from how we think it should behave.

To accelerate this critical matching of material properties to the needs of the press/die, leading-edge material suppliers and press shops have begun to implement virtual forming—also referred to as computerized die tryout or computerized forming-process development. The first run through such a system will confirm if the base-material properties match the die design required to meet part print. Need different material properties? Rather than wait 4 to 12 weeks for a new coil, a simple keyboard change of material properties can determine whether the new grade is a successful buyoff or failure. This virtual-evaluation process proves more critical for higher-strength materials, including the new types of advanced high-strength steels.

Here I have presented reasons why our industry has been slow to match material properties to the needs of the press/die. Unfortunately, adoption of virtual forming has been even slower. The concept of a computer being more accurate than 30 years of on-the-floor experience is a hard sell—this explains why OEM/Tier 1 stampers and major mills comprise the bulk of virtual-forming users. MF